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Date: Mon, 10 Apr 2017 11:29:39 +0100
From: Mark Rutland <mark.rutland@....com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Mathias Krause <minipli@...glemail.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andy Lutomirski <luto@...capital.net>,
	Kees Cook <keescook@...omium.org>,
	Andy Lutomirski <luto@...nel.org>,
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
	Hoeun Ryu <hoeun.ryu@...il.com>, PaX Team <pageexec@...email.hu>,
	Emese Revfy <re.emese@...il.com>,
	Russell King <linux@...linux.org.uk>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: Re: Re: [RFC v2][PATCH 04/11] x86: Implement
 __arch_rare_write_begin/unmap()

On Fri, Apr 07, 2017 at 04:45:26PM +0200, Peter Zijlstra wrote:
> On Fri, Apr 07, 2017 at 12:51:15PM +0200, Mathias Krause wrote:
> > Why that? It allows fast and CPU local modifications of r/o memory.
> > OTOH, an approach that needs to fiddle with page table entries
> > requires global synchronization to keep the individual TLB states in
> > sync. Hmm.. Not that fast, I'd say.
> 
> The fixmaps used for kmap_atomic are per-cpu, no global sync required.

That might be fine for x86, but for some architectures fixmap slots and
kmap_atomic mappings happen to be visible to other CPUs even if they're
not required to be.

Using an mm solves that for all, though.

Thanks,
Mark.

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